Field of the Invention
The present invention relates to a fuel cell system and a start up control method for the fuel cell system. In the fuel cell system, scavenging of at least one of a fuel gas flow field and an oxygen-containing gas flow field is performed using a scavenging gas such as air at the time of stopping power generation or after stopping power generation to prepare for the start of the next operation of the fuel cell system at low temperature such as the temperature below the freezing point.
Description of the Related Art
For example, a polymer electrolyte fuel cell employs a membrane electrode assembly which includes an anode (fuel electrode) and a cathode (air electrode), and a polymer electrolyte membrane interposed between the electrodes. The electrolyte membrane is an ion exchange membrane. The membrane electrode assembly is sandwiched between a pair of separators. A fuel gas flow field is formed between the anode and one of the separators, and an oxygen-containing gas flow field is formed between the cathode and the other of the separators. In use, normally, a predetermined numbers of the membrane electrode assemblies and separators are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas flow field. The fuel gas flows through the fuel gas flow field along the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the suitably humidified electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating DC electrical energy. Further, in the fuel cell, an oxygen-containing gas such as air is supplied to the oxygen-containing gas flow field, and the oxygen-containing gas flows along the cathode for reaction. At the cathode, hydrogen ions from the anode combine with the electrons and oxygen to produce water. Water is also retained at the anode due to back diffusion from the cathode or high humidification of the fuel gas.
If water at any of the electrodes becomes excessive, water clogging may occur. Thus, in the fuel cell system of this type, at the time of stopping operation of the fuel cell system, a technique of scavenging both sides of the anode and the cathode is proposed. In the anode/cathode scavenging technique, the oxygen-containing gas is supplied to the anode as well as the cathode for removing, e.g., water produced in power generation from the membrane electrode assembly or the separator in the fuel cell (see Japanese Laid-Open Patent Publication No. 2001-351666).
Further, after the fell cell system is stopped, when the outside temperature decreases, and operation of the fuel cell system is started at low temperature such as the temperature below the freezing point, before warming up the fuel cell, the ignition switch may be turned off by an operator such as a driver and a stop signal may be outputted from the ignition switch. Therefore, if operation of the fuel cell system is stopped after operation of the fuel cell system is started at the temperature below the freezing point, and power generation is performed for a short period of time, stated otherwise, if operation of the fuel cell system is stopped by the driver's operation in a short period of time after operation of the fuel cell is started at the temperature below the freezing point, it has been found that, in some cases, the fuel cell system becomes unstable due to the sufficient activity of the electrolyte membrane.
In Japanese Patent Application No. 2005-307193, the applicant of the present application proposes a technique for eliminating the instability beforehand to reliably starting the next operation stably, as the two stage scavenging process technique or the three stage scavenging process technique.
In the two stage scavenging process technique, or the three stage scavenging process technique, even if there is a request to stop operation of the fuel cell system after power generation for a short period of time at low temperature, when the request is received, it is possible to reliably perform the scavenging process, and it is possible to reliably restart the next operation of the fuel cell system at low temperature such as the temperature below freezing point.
In the technique disclosed in Japanese Laid-Open Patent Publication No. 2001-351666 or in the technique proposed in Japanese Patent Application No. 2005-307193, after the stop signal from the ignition switch is received, in the middle of the scavenging process of supplying the scavenging gas such as the oxygen-containing gas to the anode, in order to restart operation of the fuel cell, a start up signal may be outputted from the ignition switch again.
However, in the case of Japanese Laid-Open Patent Publication No. 2001-351666, after the stop signal from the ignition switch is received, in the middle of the scavenging process of supplying the scavenging gas such as the oxygen-containing gas to the anode, if operation of the fuel cell is restarted by supplying the fuel gas to the anode immediately after reception of the start up signal from the ignition switch, since the gas replacement by the fuel gas is not performed accurately at the anode, the desired power generation stability at the time of restarting operation may not be obtained disadvantageously.
Further, during the second stage scavenging process and the three stage scavenging process after the stop signal from the ignition switch is received, if the start up signal is outputted again from the ignition switch, and operation of restarting the fuel cell is awaited until these scavenging processes are finished, since it takes considerable time, the operator may feel a sense of discomfort.